Trishear Fault-propagation Folding Research Articles

Effect of inherited structural highs on the structure and kinematics of the South Dezful Embayment, SW Iran

AbstractUnderstanding the structural evolution of the South Dezful Embayment (SDE) in the Zagros Fold-and-Thrust Belt (ZFTB) is significant for hydrocarbon exploration and production. The structural evolution of this area has been controlled by the reactivation of basement structures related to the oblique convergence along the ZFTB. In this work, we study the effect of the pre-contractional interaction between the basement step and overlaid salt layer superimposed by the Zagros shortening on the structural style and evolution of the sedimentary basin in the SDE. We use multidisciplinary approaches involving surface and subsurface data and analogue models. Disharmonic folds with small wavelength formed over the Kharg-Mish Palaeo-high (KMPH) where the sedimentary cover is thinner. On the other hand, faulted detachment folds with large wavelengths developed in the adjacent depocenters. Furthermore, the KMPH affected the geometry and kinematics of the frontal part of the Zagros Belt, which is characterized by the Mountain Front Flexure (MFF) topographic step. Long-lasting active deformation of the sedimentary cover over the frontal ramp of the KMPH developed the South Izeh Promontory (SIP). Localized contraction in the SIP was accommodated temporally by trishear fault-propagation folding and resulted in > 5 km elevation difference between this promontory and two adjacent local basins. Supporting scaled analogue models show that structural evolution, folding and along-strike variations in fold style of the SDE have been controlled by thickness variations of the sedimentary cover and the geometry of the inherited pre-salt structure. Based on the results of this study, the effect of inherited structural highs on the structure and kinematics of the SDE provide important insights for hydrocarbon entrapment, migration and exploration.

Subsurface Modeling of Digboi‒Dilli Ghat Oil Bearing Structures (India) Using 2D Move

The first discovery of commercial hydrocarbon in India was in 1889 in a structural trap at Digboi in Assam. Ever since the discovery, geologists have mapped and modeled the subsurface structure of this 50 km long anticlinal fold-thrust belt. However, never before, the existence of a hanging wall syncline north of Jaipur anticline, near the Kusijan oil and gas field, was considered. In this paper, we have proposed a structural model for the regional fold-thrust structures that successfully explains the existence of this syncline adjacent to the Naga thrust. Trishear Fault Propagation folding followed by a complex breakthrough pattern of the thrust at later stages is proposed as most suitable model to explain the outcrop pattern and sub-surface data. We have prepared two northwest-southeast trending balanced structural cross sections using this kinematic model. Total shortening percentages across the southern and northern cross sections are estimated to be 44 and 30 respectively. A complementary play with locations of prospective boreholes is suggested. Based on the dip data in the hanging wall structures of Naga thrust, we predict medium- to small-scale concealed imbricate structures in the sub-thrust. These zones of local thrust imbrications may prove to be prospective leads for concealed petroleum traps.

Effects of fault slip distribution on the geometry and kinematics of the southern Junggar fold-and-thrust belt, northern Tian Shan

At the foreland edge of fold-and-thrust belts (FTBs), tectonic shortening could either be accommodated by thick-skinned faulting and/or structural wedges developed near range-fronts, or be transferred along detachment layers further into basins, forming thin-skinned structures. Although the geometry and kinematics of each structure have been well-investigated, studies that characterize slip distribution along a specific fault as well as the effects of variations in magnitude of fault displacement on the development of fault-related folds are still lacking. In this study, we present a case study of the southern Junggar Thrust (SJT), a seismically-active fault developed in the southern Junggar FTB, northern Tian Shan with an overall vergence to the north. The results of structural interpretations of seismic reflection profiles combined with the age constraints provided by previous magnetostratigraphic studies demonstrate that: 1) The kinematics of the SJT vary along strike from purely structural wedging to classical fault-bend folding. Besides these end-member models, intermediate cases exist between them, in which fault slip is partitioned by two-stage structural deformation, i.e., fault-bend folding subsequent to structural wedging at the range front. As a result, part of the displacement is transferred northward, forming the Tugulu anticline. 2) Along-strike variations in displacement along the Tugulu fault (frontal fault of the SJT) also affect the geometry and kinematics of the Tugulu anticline, varying from detachment folding, trishear fault-propagation folding with a pure-shear fault-bend folding back-limb, and classical trishear fault-propagation folding to fault-bend folding with increasing displacement. 3) The SJT has an average Quaternary slip rate of 3.9 ± 0.4 mm/yr, serving as a principal structure accommodating tectonic shortening in the southern Junggar FTB. The results of this study are helpful for seismic hazard assessments of the study area, which may also shed light on structural evolution of fault-related folds in other FTBs.

Jabal Hafit anticline (UAE and Oman) formed by décollement folding followed by trishear fault-propagation folding

Creating three-dimensional (3D) models to replicate geological structures is crucial in discerning the geometry and kinematics of an orogen. The Jabal Hafit anticline, in the foreland of the Al Hajar Mountains, extends through Oman and the United Arab Emirates and is a relatively simple and well exposed structure. Surprisingly, previous studies of this anticline have presented conflicting interpretations regarding the strain field and the timing of deformation. In this study a structural 3D geological model of the Jabal Hafit anticline is constructed and demonstrates that the anticlines geometry can be reproduced by a two-phase model: (1) flexural slip folding, followed by (2) trishear fault-propagation folding. The trishear mode occurred due to a west-dipping backthrust that propagated from the décollement, to accommodate WSW–directed shortening. The different geometries of the various biostratigraphic layers indicate that anticline growth occurred during the late Oligocene to early–middle Miocene. This timing supports the recently established age for uplift of the Al Hajar Mountains.

A trishear model for the deformation of the Sudbury Igneous Complex, Canada

The Sudbury Igneous Complex (SIC), Canada, is an impact-induced layered sheet of crystalline rocks deformed into an asymmetrical fold basin, the Sudbury Basin. The basin geometry at depth is largely unknown as few attempts were made to quantify displacement and rotation of layer contacts. We propose that the dip of layer contacts and foliation surfaces in the southern SIC can be approximated by trishear fault propagation folding. Trishear deformation accounts for: (1) angular discordances between upper and basal SIC contacts, (2) local overturning of the SIC, (3) progressive steepening of foliation surfaces from NW to SE, (4) strain gradient in the Sudbury Basin sedimentary rocks, and (5) thickness variations in SIC layers. Moreover, the South Range Shear Zone, a zone of moderately strong metamorphic fabrics, coincides with the surface manifestation of the proposed trishear zone.We demonstrate the use of structural data together with forward kinematic modelling to identify the strain distribution within the SIC, rotation of SIC contacts and thickness changes of SIC layers. Backward kinematic modelling provides information on the initial geometry of the SIC and is used to restore the shape of the igneous sheet, showing that the SIC was shortened by approximately 10 km in NW-SE diameter. Most of the shortening was accommodated by 40% reduction in the thickness of the upper SIC layer.

High precision analysis of an embryonic extensional fault-related fold using 3D orthorectified virtual outcrops: The viewpoint importance in structural geology

Image-based 3D modeling has recently opened the way to the use of virtual outcrop models in geology. An intriguing application of this method involves the production of orthorectified images of outcrops using almost any user-defined point of view, so that photorealistic cross-sections suitable for numerous geological purposes and measurements can be easily generated. These purposes include the accurate quantitative analysis of fault-fold relationships starting from imperfectly oriented and partly inaccessible real outcrops. We applied the method of image-based 3D modeling and orthorectification to a case study from the northern Apennines, Italy, where an incipient extensional fault affecting well-layered limestones is exposed on a 10-m-high barely accessible cliff. Through a few simple steps, we constructed a high-quality image-based 3D model of the outcrop. In the model, we made a series of measurements including fault and bedding attitudes, which allowed us to derive the bedding-fault intersection direction. We then used this direction as viewpoint to obtain a distortion-free photorealistic cross-section, on which we measured bed dips and thicknesses as well as fault stratigraphic separations. These measurements allowed us to identify a slight difference (i.e. only 0.5°) between the hangingwall and footwall cutoff angles. We show that the hangingwall strain required to compensate the upward-decreasing displacement of the fault was accommodated by this 0.5° rotation (i.e. folding) and coeval 0.8% thickening of strata in the hangingwall relatively to footwall strata. This evidence is consistent with trishear fault-propagation folding. Our results emphasize the viewpoint importance in structural geology and therefore the potential of using orthorectified virtual outcrops.

Combining multiple quantitative structural analysis techniques to create robust structural interpretations

Carefully selected 2D transects contain an abundance of structural information that can constrain 3D analyses of petroleum systems. Realizing the full value of the information in a 2D transect requires combining multiple, independent structural analysis techniques using fully interactive tools. Our approach uses quantitative structural geologic software that instantaneously displays structural computations and analyses, eliminating time-intensive manual measurements and calculations. By quickly testing multiple hypotheses, we converged on an optimal solution that is consistent with available data. We have combined area-depth-strain (ADS) analysis, structural restoration, and forward modeling of a structural interpretation of a fault-propagation fold in the Niger Delta. These methods confirmed the original interpretation and furthermore quantified displacement, strain, detachment depth, and kinematic history. ADS analysis validated the interpreted detachment depth and revealed significant layer-parallel strain (LPS) that varied systematically with stratigraphic depth. The stratigraphic distribution of the LPS was diagnostic of structural style and, in this example, discriminated against fixed-axis and constant-thickness fault-propagation folding. A quantitative forward model incorporating backlimb shear and trishear fault-propagation folding accurately reproduced folding and faulting in the pregrowth section and folding in the growth section. The model-predicted strain distributions were consistent with those from ADS analysis. The highest local strains on the back limb of the structure were spatially coincident with two backthrusts, which accommodated these strains. Animations of a more complete model including the backthrusts revealed that the backthrusts formed sequentially as rock passed through the main fault bend.

TrishearCreator: A tool for the kinematic simulation and strain analysis of trishear fault-propagation folding with growth strata

TrishearCreator is a platform independent web program constructed in Flash, which enables fold modeling, numerical simulation of trishear fault-propagation folding and strain analysis, etc. In the program, various types of original strata, such as folds and inclined strata can be easily constructed via adjusting shape parameters. In the simulation of trishear fault-propagation folding, growth strata and strain ellipses are calculated and displayed simultaneously. This web-based program is easy to use. Model parameters are changed by simple mouse actions, which have the advantage of speed and simplicity. And it gives an instant visual appreciation of the effect of changing the parameters that are used to construct the initial configuration of the model and the fold-propagation folding. These data can be exported to a text file, and be shared with other geologists to replay the kinematic evolution of structures using the program.

Application of trishear fault-propagation folding to active reverse faults: examples from the Dalong Fault, Gansu Province, NW China

Determining accurate fault slip rates at 1 ka to 1 Ma timescales requires well-constrained palinspastic reconstructions of dateable geomorphic and/or geologic markers. Although general kinematic models have been developed to simultaneously reconstruct both bedrock (e.g. bedding and fault attitudes) and neotectonic markers (e.g. strath terraces) along active strike-slip and thrust faults, it is not clear if these models can also account for deformation along steeply dipping (>45°) reverse faults. To address this problem, we have investigated the active, ∼50° dipping, Dalong reverse fault system. This ∼40-km-long fault system forms part of the Aksai restraining stepover along the active, left-slip Altyn Tagh Fault in northwestern China. Our geometric and kinematic analyses show that conventional fault-bend fold models cannot satisfy the steeply-dipping fault geometry we observe in the bedrock record. Likewise, standard fault-propagation fold models fail to match our measurements of a set of fluvial terraces. However, by expanding the trishear model of fault-propagation folding to track both bedrock and neotectonic markers, we are able to match both sets of records. In particular, we have developed trishear kinematic models for two sites (Liuchenzi and Qingyazi) using the numerical modeling program, Fault/Fold v.5.0. This work indicates that an important implication of active trishear fault-propagation folding is that terrace deformation extends for over 1 km on either side of the fault trace. Thus, to accurately measure the total magnitude of vertical separation between matching terraces in the hanging wall and footwall, terrace profiles across active reverse faults must extend 1–2 km on either side of this zone of deformation.

Pseudo 3-D modeling of trishear fault-propagation folding

Basement structures, to which trishear fault-propagation models have most successfully been applied, are commonly three-dimensional folds formed at the tip-line of a fault. We present here a ‘pseudo-3D’ trishear model in which various parameters are permitted to vary along strike and oblique-slip can be modeled. These variations may be combined in an infinite number of ways, facilitating the simulation of many real structures. A thrust changing from blind to emergent can be produced by a change in the slip or propagation-to-slip (P/S) along strike. Folds with forelimbs changing from overturned to upright along strike can be modeled either by changing the slip, P/S or trishear angle. Also some minor folds perpendicular or oblique to the main structure can result from changes in the trishear angle or fault angle along the strike. Models including growth strata show that it is practically impossible to distinguish between growth and pre-growth strata using the map patterns. As a field test, we have modeled the oblique slip East Kaibab monocline, demonstrating a good fit between the field observations and model predictions.

Numerical modeling of trishear fault propagation folding

In contrast to kink band migration modeling methods, trishear numerical models produce fault propagation folds with smooth profiles and rounded hinges. Modeled fold hinges tighten and converge downward, within a triangular zone of distributed deformation which is focused on the fault tip. Such features have been reported from field studies and are also seen in analogue models of compressional deformation. However, apart from its initial application to Laramide folds, little quantitative work has been undertaken on trishear fault propagation folding in other settings. In addition, no study has been undertaken into the growth strata which might be associated with such structures. This paper uses an equivalent velocity description of the geometric model of trishear, together with models of erosion and sedimentation, to investigate trishear fault propagation folding of both pregrowth and growth strata. The trishear model is generalized to include a variety of fault propagation to slip ratios and fault propagation from a flat decollement. The models show continuous rotation of the forelimb with the characteristic development of cumulative wedges within growth strata. When total slip on a structure is high, the model predicts overturned pregrowth and growth strata. During the initial stages of deformation, beds in the forelimb thicken but later thin when they become steep or overturned. The effect of variations in fault propagation to slip ratios on two‐dimensional finite strain in the models is assessed by the use of initially circular strain markers. High fault propagation to slip (p/s) ratios lead to narrow zones of high finite strain, while lower p/s ratios lead to more ductile deformation and broader zones of high strain. In all cases, hanging wall anticlines and footwall synclines originate as early ductile folds which are later cut by the propagating fault. Modeled structures are compared with natural examples.

Trishear fault-propagation folding

Previous models of fault-propagation folding used kink-band geometries to approximate folding in front of propagating thrusts. However, kink-band kinematics cannot replicate the curved fold surfaces and complex strain patterns in natural and experimental fault-propagation folds, which also occur in front of steeper reverse and normal faults. Fault-propagation fold hinges tighten and converge downward, forming a triangular zone of penetrative deformation focused on the tip of the propagating fault. The downward convergence of deformation in fault-propagation folds can be modeled as triangular shear zones. Trishear, here defined as distributed, strain-compatible shear in a triangular (in profile) shear zone, provides an alternate kinematic model for fault-propagation folds. Trishear is analogous to simple shear in a tabular shear zone except that area balance in a triangular shear zone requires curved displacement oblique to the fault slip direction. Incremental computer models of trishear folding can replicate many geometric features of fault-propagation folds, including variably curved fold hinges, downward-tightening fold surfaces, heterogeneous strains, and multiple fault-propagation trajectories.

Electrolytic Medication with Special Reference to Ionic Chemistry

Creating three-dimensional (3D) models to replicate geological structures is crucial in discerning the geometry and kinematics of an orogen. The Jabal Hafit anticline, in the foreland of the Al Hajar Mountains, extends through Oman and the United Arab Emirates and is a relatively simple and well exposed structure. Surprisingly, previous studies of this anticline have presented conflicting interpretations regarding the strain field and the timing of deformation. In this study a structural 3D geological model of the Jabal Hafit anticline is constructed and demonstrates that the anticlines geometry can be reproduced by a two-phase model: (1) flexural slip folding, followed by (2) trishear fault-propagation folding. The trishear mode occurred due to a west-dipping backthrust that propagated from the décollement, to accommodate WSW–directed shortening. The different geometries of the various biostratigraphic layers indicate that anticline growth occurred during the late Oligocene to early–middle Miocene. This timing supports the recently established age for uplift of the Al Hajar Mountains.